Owing to the explosive increase of the number of Internet users and the increased capacities and the diversification of contents, it is hoped that the transmission capacity of wireless communication is increased. As high-capacity wireless transmission, the utilization of a millimeter waveband in which the number of commercial wireless stations is small and a wide frequency band is easily secured is suitable. Compared with a narrowband communication device that uses a carrier system, a wireless communication device, which uses an impulse system, has a characteristic that a local oscillator and a mixer are not necessary, and hence the configuration of a wireless section is simple and the cost of the wireless section is low. Therefore, the wireless communication device is hoped as a tool for realizing high-capacity wireless transmission that has a transmission rate of more than 10 gigabits per second (Gbps).
For example, FIG. 1 is a diagram illustrating an impulse radio communication system that uses a millimeter waveband and, for example, has a transmission rate of 10 Gbps.
As illustrated in FIG. 1, the impulse radio communication system that uses the millimeter waveband includes a transmission device 100 and a reception device 105. The transmission device 100 includes a short pulse (impulse) generator 101, a bandpass filter 102, a transmission amplifier (amp) 103, and an antenna 104. The reception device 105 includes an antenna 106, a reception amplifier (amp) 107, and a detector 108.
In the transmission device 100, on the basis of a data signal that is input from a baseband section and has a bit rate of, for example, 10 Gbps, the short pulse generator 101 generates a signal that has an infinitesimal half-value width (pulse width) and is called an impulse. The impulse output from the short pulse generator 101 includes energy that ranges from direct current to a high frequency. As illustrated in FIG. 2, the bandpass filter 102 extracts only a frequency component, used for communication, from a wide band frequency component included in the impulse. Accordingly, the output of the bandpass filter 102 is modulated as a wave packet that oscillates in the vicinity of the center frequency of a passband. The transmission amplifier 103 amplifies the wave packet output from the bandpass filter 102 so that the power of the wave packet reaches a certain level. The antenna 104 transmits the amplified wave packet as a millimeter-wave signal to the air. On the other hand, in the reception device 105, the reception amplifier 107 amplifies a weak millimeter-wave signal received by the antenna 106, and the detector 108 detects the envelope of the millimeter-wave signal to decode the millimeter-wave signal as a data signal.
An impulse communication device that uses a millimeter waveband includes a transmission device 100 and a reception device 105, and transmits and receives a signal to and from a communication device. In some cases, the impulse communication device includes the transmission device 100 and the reception device 105 separately. However, in other cases, the impulse communication device includes a common antenna used for transmission and reception and a switch which switches the connection state of the antenna so that the antenna is connected to one of the transmission amplifier 103 and the reception amplifier 107. Hereinafter, a transmission device included in a communication device used for transmission and reception is called a transmission section, and a reception device included in the communication device is called a reception section.
For example, in an impulse communication system in which a millimeter waveband used for an outdoor fixed-line communication is utilized, it is desirable to secure a wide dynamic range in the entire system, in order to cover a transmission path fluctuation due to a weather condition or the like.
FIG. 3 is a diagram illustrating changes in a signal-to-noise (S/N) ratio, due to an outdoor situation, in a case in which an impulse communication system that uses a millimeter waveband, which has a frequency of 85 gigahertz (GHz) and a bandwidth of 10 GHz, and has transmission power of 100 milliwatts (20 dBm), a transmission and reception antenna gain of 50 dBi, and a noise figure of 5 decibel (dB) is used for an outdoor fixed-line communication. In FIG. 3, a horizontal axis indicates a precipitation amount of millimeters per hour (mm/h), a vertical axis indicates a S/N ratio (dB), “A” indicates a clear weather state, “B” indicates a rainfall state that nearly corresponds to a drizzle, “C” indicates a rainfall state that corresponds to a heavy rain, “D” indicates a rainfall state that corresponds to a sheeted rain that occurs less commonly during a year, and “1 km” and “3 km” indicate communication distances in kilometers.
With reference to FIG. 3, it turns out that, when data is transmitted using a bit rate of 10 Gbps over a distance of 3 km using a millimeter wave the band of which is 80 GHz-90 GHz, a fluctuation of more than or equal to 100 dB in a space propagation loss occurs between a clear weather condition and a sheeted rain condition in which a precipitation amount is 100 mm per hour.
A communication system is requested to allow a communication path to be secured even in the sheeted rain. Accordingly, the communication device is requested to have a dynamic range of more than or equal to 100 dB. In a usual impulse communication device that uses a millimeter waveband, a dynamic range has been secured by causing the gain of a transmission amplifier or a reception amplifier to be variable.
FIG. 4A is a diagram illustrating the configuration of the transmission device in which the gain of the transmission amplifier is variable. FIG. 4B is a diagram illustrating the configuration of the reception device in which the gain of the reception amplifier is variable.
The configuration of the transmission device illustrated in FIG. 4A corresponds to a configuration in which, in the transmission device 100 in FIG. 1, the transmission amplifier 103 is replaced with a variable gain transmission amplifier 103′. The configuration of the reception device illustrated in FIG. 4B corresponds to a configuration in which, in the reception device 105 in FIG. 1, the reception amplifier is replaced with a variable gain reception amplifier 107′.
In a system in which the transmission device and the reception device, illustrated in FIGS. 4A and 4B respectively, are used, for example, while the gains of the variable gain reception amplifier 107′ and/or the variable gain transmission amplifier 103′ are individually set to maximum in a sheeted rain, the gains of the variable gain reception amplifier 107′ and/or the variable gain transmission amplifier 103′ are individually reduced in clear weather.
However, typically, a relationship between a dynamic range and performances such as a low noise property, a high output property, and a broadband property is a trade-off relationship. Therefore, if a variable gain range is enlarged, it is necessary to sacrifice these properties. In this way, in the communication device that uses a millimeter waveband, including the impulse radio communication device, it has been difficult to secure a sufficient dynamic range.
An example of the related art is Japanese Unexamined Patent Application Publication No. 2008-205733, and R. Yamaguchi, et al. “10-Gbit/s MMIC Wireless Link Exceeding 800 Meters” RWS2008 Digest, pp. 695-698.